The present invention relates to a method of improving adhesion between a metal and an organic dielectric material when the metal is electrolessly deposited on the organic dielectric layer. Specifically, the invention relates to the formation of the dielectric material layer exhibiting phase separated morphology and rough surface topography of at least two phases of the dielectric material and the utilization of a dispersed catalyst seed layer to improve the adhesion of subsequently electrolessly deposited metal onto the dielectric layer. Quite specifically, the invention concerns selective electroless deposition of copper, nickel, cobalt, gold, palladium or any other electroless metal or metal alloy onto polyimide with improved adhesion.
Electroless copper deposition on polyimide principally using palladium as a seed layer has important applications in high-end microelectronics technology. However, improving the adhesion between the deposited copper and the polyimide is a major problem.
U.S. Pat. No. 3,370,973 describes a method in which a ternary solution of a polar solvent such as NMP (normal methyl pyrrolidone), a Pd bearing salt and polymide is formed. The solution is cast as a very thin film, which upon spinning and drying has 25 to 50 .ANG. particles of Pd colloid dispersed uniformly throughout the layer. If the amount of Pd salt is low (below 0.01% in the ternary solution and in the range between 0.001 to 0.0001% in the final film), after a conventional polyimide curing step, upon exposure to an alkaline solution, or to a plasma of air, nitrogen or the like, the Pd colloidal particles are exposed and act as catalytic sites for electroless copper (metal or metal alloy) deposition. Since many electroless baths are alkaline, it is possible to expose the Pd colloidal dispersion in the polyimide in situ, in the bath.
Since the colloidal particles are separated by a distance of 400 to 600.ANG. or more and the particles themselves are only 25 to 50.ANG. and the dielectric properties of the Pd containing polymer film is not affected and current leakage between plated copper lines shows no surface conductivity as a result of the presence of the particles. Surface leakage and conductivity can be increased by increasing the loading of the solvent-polyimide mixture with additional Pd salt. If desired, the loading can be increased to the level of nearly metallic conductivity.
Adhesion of electroless copper or nickel on Pd doped polyimide which shows no visible presence of Pd (very low concentration) is described in U.S. Pat. No. 3,370,973 and the film adhesion strength was equivalent to approximately 30 to 50 g/mm. The adhesion was postulated to be primarily due to mechanical interlocking of Pd particles.
Selective plating through resist masks and activation by forming gases after resist exposure, alkaline treatment or forming gas plasma ashing is described in IBM TDB vol. 13, October 1970, p. 1199 and in IBM TDB vol. 25, December 1982, pp. 3336-38.
U.S. Pat. Nos. 4,618,568 and U.S. Pat. No. 5,032,488 describe the use of radiation to deposit and/or reduce Pd-salt for surface activation that may be used for electroless plating.
U.S. Pat. Nos. 4,948,707 and 4,701,351 use an alkaline solution (NaOH) to hydrolyze an organic surface for Pd-salt deposition. In U.S. Pat. No. 4,948,707 the surface is roughened ex-situ, by the addition of a separate roughening step.
U.S. Pat. No. 5,045,436 and IBM TDB vol. 33, February 1991, p. 418-421 describe activation by thermal degradation of Pd-salt. However, both references fail to describe the use of a precursor polyimide blend.
U.S. Pat. No. 4,940,609 describes surface modification using a chemical process involving quaternary amine salts. That is, chemical means is used to achieve adhesion rather than physical properties related to mechanical interdigitation at a 100-1000 .ANG. scale.
In accordance with the teachings of the present invention, a morphologically and topographically rough surface is spontaneously formed, without any ex-situ roughening process. The rough surface provides enhanced adhesion, not previously obtained by physical roughening of organic dielectric layers.